The discovery that epoxides (alkylene oxides) rearrange to give allylic alcohols in the presence of basic lithium phosphate catalysts sparked many efforts to improve catalyst lifetime, productivity, and selectivity. Allyl alcohol, the simplest allylic alcohol, is produced by isomerizing propylene oxide. Allyl alcohol is converted to useful allyl derivatives (diallyl phthalate, diallyl ether, diethylene glycol bis(allyl carbonate), etc.) or is converted to 1,4-butanediol and its derivatives.
Two general isomerization processes are known: the vapor-phase isomerization process (see, for example, U.S. Pat. Nos. 3,044,850, 4,720,598, and 5,262,371), and the slurry-phase process (see, for example, U.S. Pat. No. 3,274,121). In the vapor-phase process, the epoxide is passed through supported or unsupported lithium phosphate at elevated temperatures, and the allylic alcohol is recovered and purified by distillation. A drawback of many vapor-phase processes: nonvolatile by-products accumulate on the catalyst surface over time and rapidly stifle catalyst activity.
The slurry-phase process, which is practiced commercially, was developed to overcome the catalyst deactivation problems of the vapor-phase process. In the slurry-phase process, lithium phosphate is suspended in a high-boiling oil. During the reaction, a portion of the catalyst suspension is continuously removed and centrifuged to separate the tar-containing oil from the catalyst. Tars are distilled from the oil, the lithium phosphate is washed with acetone, and the purified catalyst components are recycled to the reactor. Problems with the slurry-phase process include catalyst loss and high oil consumption.
A problem common to both the vapor-phase and slurry-phase processes is low selectivity to the allylic alcohols. Propylene oxide, for example, isomerizes to allyl alcohol, but also gives significant amounts of propionaldehyde, acetone, and 1-propanol.
Recently, we described (U.S. Pat. Nos. 5,262,371 and 5,292,974) improved epoxide catalysts for vapor-phase isomerization which comprise lithium phosphate and a neutral inorganic support, particularly alkali metal-exchanged zeolites. These catalysts give high allyl alcohol selectivities and good productivity in propylene oxide isomerizations. We indicated that silica can be used as a neutral inorganic support.
Gago et al. (Spanish Patent No. 2,036,449) teach silica-supported lithium phosphate catalysts for isomerizing propylene oxide to allyl alcohol. The catalysts are prepared by reacting aqueous sodium phophate with an aqueous mixture of lithium and sodium hydroxides in the presence of silica that has a high surface area, followed by washing and drying of the resulting precipitated catalyst. The reference teaches that high-surface-area silicas are preferred, but says nothing more about the silica.
Our initial results with ordinary silica catalyst supports indicate that even high-surface-area silicas give low allylic alcohol selectivities in epoxide isomerizations. When propylene oxide is isomerized, for example, the selectivity to allyl alcohol is somewhat low, and significant levels of by-products (propionaldehyde, acetone, 1-propanol) form.
Improved epoxide isomerization catalysts are needed, particularly those useful in a vapor-phase isomerization process. Preferred catalysts would give high selectivity to the allylic alcohols and a reduced proportion of non-selective by-products.